There is not much difference between Havanese boys and girls if spayed/neutered before puberty.
Boy will never raise his leg and mark the territory. 
He will always pee like a girl (squat) if neutered before hormones kick in.

However, there is a saying: “Your girls love you but your boys are IN LOVE with you.”

Girls usually bond with one member of the family (usually female of the house who is feeding and taking care of the dog) whereas boys love everyone equal. 

Boys are usually more playful than girls thus highly recommended for families with kids.

Boys are more lap dogs compared to girls. Most girls will jump in your lap, give you a kiss and jump down. Most boys will sit in your lap forever.

I will copy and paste some info from Canada Havanese fanciers website:

If your Havanese is to be a family pet or companion dog then in all likelihood it will be spayed or neutered which removes many of the above behaviors associated with hormones in both genders. In altered Havanese, the differences tend to be much less pronounced. Spayed and neutered Havanese make the best family companions. Overall, whether altered or not, Havanese males may be more loving and easy going while females may be more demanding and temperamental. This is not to say that one makes a better pet than the other but simply that this is a generality and something to keep in mind when evaluating puppies for a suitable family companion, particularly if children are involved. Of course, individual differences within a litter or between breedlines may also contribute to temperament variations in both sexes. Some people have a personal preference and gravitate towards one sex over the other for assorted reasons. If you have a preference, by all means discuss it with your breeder. Havanese are all very sweet but also very individual. Each is precious and wonderful and deserves the best home possible.